The Fourth International Conference on the Molecular Biology and Pathogenesis of the Clostridia will be held April 26-30, 2003 at the Marine Biological laboratories in Woods Hole, MA. This meeting will attract about 150 leading international clostridial researchers from universities, government laboratories, and commercial organizations. The conference will focus on new developments regarding the virulence and basic biology of the pathogenic clostridia, which remain major causes of natural enteric, histotoxic, and neurotoxic disease in humans and economically-important domestic animals. In addition, toxins produced by the pathogenic Clostridia have become major biodefense concerns, with botulinum toxins and C. perfringens epsilon toxin listed as Class A and B select agents, respectively. The conference's goal is two-fold: i) providing a common forum to discuss recent research findings in order to stimulate future progress, and it) training the next generation of clostridial researchers. To accomplish these purposes, the meeting consists of 10 sessions combining talks and posters. The first session will address the clostridial enterotoxins, which cause symptoms of several extremely important noscomial and foodborne diseases. The following session will concern the epidemiology, diagnosis and treatment of clostridial infections. Promising new practical medical and biotechnology applications of the pathogenic clostridia (and their toxins) will then be discussed in the third conference session. Session Four will then specifically focus on membrane-active clostridial toxins, which are so important for histotoxic infections such as gas gangrene. In the fifth session, the economically important clostridial veterinary diseases will be discussed. Exciting new insights provided by several clostridial genome projects will be presented in the 6th session, along with new information about clostridial mobile genetic elements, which are important for virulence. The seventh session will have considerable biodefense relevance since it includes the clostridial neurotoxins, including botulinum toxins. How clostridia regulate virulence gene expression will be the topic of the 8th session, to be followed in session 9 by a discussion of the linkage between clostridial physiology and virulence. The conference will conclude with an in-depth analysis of interactions between the pathogenic clostridia and their hosts in Session 10.